As the demand on the device integration is gradually increased, the size of the transistor mounted on the semiconductor chip becomes smaller and smaller. However, the size reduction may impair the performance of the transistor, for example the on-state current (Ion) of the transistor is too low and the gate leakage current (Jg) of the transistor is too high. Therefore, there is a need of effectively enhancing the performance of the transistor without increasing the size thereof.